The present invention generally relates to optical semiconductor devices and more particularly to a GaN-family laser diode producing blue to ultraviolet radiation and a fabrication process thereof.
Laser diodes, light-emitting diodes and photodiodes are optical semiconductor devices used extensively in the field of optical telecommunication, optical information processing, recording of information, and the like.
In the case of laser diode, there is a demand, particularly in the field of optical information recording, for a laser diode operable in the optical wavelength band of blue to ultraviolet radiation for increasing the recording density. It should be noted a laser diode oscillates generally in the optical wavelength band of red to infrared radiation. Further, there is a demand for a photodiode operable in such a short optical wavelength band.
Conventionally, GaN, having a large bandgap, has been recognized as a promising material for constructing an optical semiconductor device such as a laser diode or photodiode that operates in the foregoing blue to ultraviolet wavelength band. A light-emitting diode using a GaN crystal for the active layer thereof is already put into practical use. Further, a laser diode having a double heterostructure of GaN/InGaN/GaN is already known. By incorporating an appropriate impurity element into the GaN crystal, it is also possible to cause the laser diode to oscillate in the visible wavelength band of green radiation.
It should be noted that GaN has a Wurtzite structure belonging to the hexagonal crystal system, and the preparation of a single crystal substrate of GaN is difficult. Thus, the optical semiconductor devices that use GaN for the active layer have been constructed on the c-surface of a sapphire (Al2O3) substrate, which also belongs to the hexagonal crystal system. Thereby, the GaN active layer is grown on the foregoing c-surface of sapphire substrate epitaxially.
FIG. 1 shows the construction of a conventional GaN-family laser diode 1 operable in the optical wavelength band of blue to ultraviolet radiation.
Referring to FIG. 1, the laser diode 1 is formed on a sapphire substrate 11 and includes a GaN buffer layer 12 formed on the substrate 11, an n-type GaN electrode layer 13 formed on the GaN buffer layer 12, and a lower cladding layer 14 of n-type AlGaN formed on the electrode layer with a composition of Al0.09Ga0.91N.
On the lower cladding layer 14, there is formed an optical waveguide layer 15 of n-type GaN, and an active layer having a multiple quantum well (MQW) structure is formed on the n-type optical waveguide layer 15 epitaxially, wherein the MQW structure includes a repetitive stacking of a unit structure of undoped InGaN quantum well layer sandwiched by a pair of undoped GaN barrier layers.
The active layer 15 is covered by an optical waveguide layer 17 of p-type GaN, and an upper cladding layer 18 of p-type AlGaN is formed on the optical waveguide layer 17 with a composition of Al0.09Ga0.91N. The upper cladding layer 18 is formed with an optical waveguide ridge 18A extending in the axial direction of the laser diode at a laterally central part thereof, and a contact layer 19 of p-type GaN is formed on the top surface of the optical waveguide ridge 18A.
The upper cladding layer 18 and the GaN contact layer 19, including both side walls of the optical waveguide ridge 18A, are covered by an insulation film 20 of SiO2, and a p-side electrode 21 is formed on the insulation film 20 in electrical contact with the GaN contact layer at the optical waveguide ridge 18A via a via-hole formed in the insulation film 20.
The foregoing semiconductor layers 14–18 form together a stacked layered structure defined by two vertical side walls W1 and W2 extending substantially vertically to the principal surface of the substrate 11. Further, there is formed an optical cavity in the stacked layered structure by a pair of mirror surfaces disposed so as to face in a direction perpendicularly to the sheet of FIG. 1.
Further, the substrate 11, buffer layer 12 and the electrode layer 13 thereon extend laterally beyond the foregoing side wall surface W2, and an n-side electrode 22 is provided on the electrode layer 13. The laser diode of FIG. 1 oscillates in the optical wavelength of 390–420 nm and has an important application in the field of high-density information recording.
The laser diode of FIG. 1, however, has a drawback in that, due to the existence of large lattice misfit of as much as 13% or more at the heteroepitaxial interface between the c-surface of the sapphire single crystal substrate 11 and the GaN epitaxial layer 12, the epitaxial layers 15–17 forming the MQW structure 16 tend to include various crystal defects with a high concentration level. Further, the laser diode 1 of FIG. 1 has a difficulty, contrary to the conventional edge-emission type, in that formation of the electrode on the bottom surface of the sapphire substrate 11 is difficult. Thereby, the construction, and hence the fabrication process of the laser diode becomes inevitably complex. In addition, the sapphire substrate used in the laser diode 1 is difficult to be cleaved, and thus, it is difficult in the laser diode 1 to form the mirror surfaces by a conventional cleaving process, contrary to the conventional edge-emission type laser diode constructed on a substrate having a zinc blende structure.
In the laser diode 1 of FIG. 1, the foregoing mirror surfaces are formed by a dry etching process, while such a process of forming the mirror surface by a dry etching process takes a substantial time. Further, the quality of the mirror surfaces thus formed is inferior, in terms of flatness and angle, to the quality of the mirror surfaces formed by a cleaving process.
It is also proposed to use a conductive SiC substrate, which also belongs to the hexagonal crystal system, in place of the sapphire substrate and form the GaN-family active layer of the optical semiconductor on such an SiC substrate. For example, the Japanese Laid-Open Patent Publication 10-135575 describes a technology of growing a GaN-family active layer on the (0001)Si surface of a 6H—SiC single crystal substrate epitaxially. It should be note that use of an SiC substrate has various advantageous features such as small lattice misfit, less than 4%, between the GaN active layer and the SiC substrate, electrical conductivity of the substrate, and excellent thermal conductivity of the substrate, which is superior to that of a sapphire substrate. Thus, by using an SiC substrate, it is possible to construct a laser diode oscillating in the optical wavelength of blue to violet radiation by using a construction similar to that of a conventional edge-emission type laser diode.
In order to construct an optical semiconductor device that uses a GaN-family active layer formed on such an SiC substrate epitaxially, it is necessary to establish a technology to form a GaN buffer layer on the SiC substrate epitaxially. Unfortunately, it is known that an epitaxial growth of a GaN layer tends to lead to an island-like growth when the growth is conducted on a SiC substrate. When such an island-like growth occurs in the buffer layer, it is difficult to form the GaN-family active layer thereon with a planarized top surface. Further, the GaN-family active layer thus formed tends to incorporate therein various crystal defects, while such crystal defects impedes the interaction occurring in the active layer between GaN and photons. Thereby, the efficiency of laser oscillation is deteriorated seriously.
It is known that the problem of island-like growth of GaN film is avoided when the SiC substrate is covered by a buffer layer of AlN or AlGaN, such that the desired epitaxial growth of the GaN active layer occurs on such a buffer layer. However, it has been not possible to provide electrical conductivity to an AlN film used for the buffer layer.
When a buffer layer of AlGaN is used, on the other hand, it is possible to provide an n-type conductivity to the buffer layer, as long as the content of Al in the AlGaN buffer layer does not exceed 40%. Thus, as long as the composition of the AlGaN layer is controlled as such, it is possible to electrically interconnect the GaN active layer with the SiC substrate via the AlGaN buffer layer. On the other hand, the condition of forming the conductive AlGaN epitaxial film on a SiC substrate with a flat and smooth top surface suitable for forming an active layer of GaN thereon, has not been explored to the date.